SUNIONSTORE for storing results in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of SUNIONSTORE command?

Given two Redis sets:
SET1 contains {"apple", "banana"}
SET2 contains {"banana", "cherry"}
What will be the number of elements in the destination set after running:
SUNIONSTORE dest SET1 SET2?

Redis

SUNIONSTORE dest SET1 SET2

A 3

B 4

C 2

D 1

💡 Hint

SUNIONSTORE stores the union of all members from the given sets into the destination key.

The union of {"apple", "banana"} and {"banana", "cherry"} is {"apple", "banana", "cherry"}, which has 3 elements.

🧠 Conceptual

intermediate

What does SUNIONSTORE do in Redis?

Choose the correct description of the Redis command SUNIONSTORE.

A Adds a member to a set.

B Deletes all members from the specified sets.

C Returns the intersection of multiple sets without storing.

D Stores the union of multiple sets into a new set key.

💡 Hint

SUNIONSTORE combines sets and saves the result.

SUNIONSTORE computes the union of given sets and stores the result in a destination key.

📝 Syntax

advanced

Which SUNIONSTORE command syntax is correct?

Identify the correct Redis SUNIONSTORE command syntax to store the union of sets 'setA' and 'setB' into 'resultSet'.

A SUNIONSTORE resultSet setA setB

B SUNIONSTORE setA setB resultSet

C SUNIONSTORE setA resultSet setB

D SUNIONSTORE resultSet (setA, setB)

💡 Hint

The first argument after SUNIONSTORE is the destination key.

The syntax is SUNIONSTORE destination_key key1 [key2 ...]. Option A follows this order correctly.

❓ optimization

advanced

How to optimize multiple SUNIONSTORE operations?

You need to store the union of three sets: set1, set2, and set3 into 'finalSet'. Which approach is more efficient?

A Run SUNIONSTORE finalSet set1, then SUNIONSTORE finalSet set2, then SUNIONSTORE finalSet set3

B Run SUNIONSTORE tempSet set1 set2, then SUNIONSTORE finalSet tempSet set3

C Run SUNIONSTORE finalSet set1 set2 set3

D Run SUNIONSTORE set1 set2 set3 finalSet

💡 Hint

SUNIONSTORE accepts multiple sets at once.

SUNIONSTORE can take multiple keys to union in one command, which is more efficient than chaining commands.

🔧 Debug

expert

Why does this SUNIONSTORE command fail?

Given the command:
SUNIONSTORE resultSet set1 set2 123
Assuming set1 and set2 are valid sets, why does this command fail?

Redis

SUNIONSTORE resultSet set1 set2 123

A Because all keys must exist before using SUNIONSTORE

B Because '123' is not a valid key name for a set

C Because '123' is interpreted as a number, causing a syntax error

D Because SUNIONSTORE only accepts exactly two sets

💡 Hint

Redis keys can be strings, but numeric strings are allowed if used consistently.

Redis keys can be any string, including numeric strings, so '123' is valid as a key name. The command does not fail for that reason. However, if '123' does not exist as a set, it is treated as an empty set, so no error occurs. The actual failure is if the key is not a set type. But since the question states failure, the only plausible reason is that '123' is not a set type key, causing a type error.

Practice

1. What does the Redis command SUNIONSTORE destination key1 key2 do?

easy

A. It merges the sets stored at key1 and key2 and stores the result in destination.

B. It deletes the sets at key1 and key2 and creates a new set at destination.

C. It returns the union of sets at key1 and key2 without storing it.

D. It copies the set at key1 to destination without merging.

Solution

1. Step 1: Understand SUNIONSTORE purpose

   The command merges multiple sets into one combined set.

2. Step 2: Identify storage behavior

   The result is stored in the specified destination key, overwriting if it exists.

3. Final Answer:

   It merges the sets stored at key1 and key2 and stores the result in destination. -> Option A

4. Quick Check:

   SUNIONSTORE merges and stores = C [OK]

Hint: SUNIONSTORE merges sets and saves result in destination [OK]

Common Mistakes:

• Confusing SUNIONSTORE with SUNION (which does not store)
• Thinking it deletes original sets
• Assuming it copies only one set

2. Which of the following is the correct syntax to store the union of sets setA and setB into resultSet using Redis?

easy

A. SUNIONSTORE setA setB resultSet

B. SUNIONSTORE resultSet setA setB

C. SUNION resultSet setA setB

D. SUNIONSTORE setA resultSet setB

Solution

1. Step 1: Recall SUNIONSTORE syntax

   The correct syntax is SUNIONSTORE destination key1 key2 ....

2. Step 2: Match keys to syntax

   Here, resultSet is destination, followed by sets setA and setB.

3. Final Answer:

   SUNIONSTORE resultSet setA setB -> Option B

4. Quick Check:

   Destination first, then sets = A [OK]

Hint: Destination key comes first in SUNIONSTORE command [OK]

Common Mistakes:

• Placing destination key last
• Using SUNION instead of SUNIONSTORE for storing
• Mixing order of keys

3. You have three sets:
setX = {1, 2, 3}
setY = {3, 4, 5}
setZ = {5, 6}
You want to create a new set combined that contains the union of all three sets. Which command correctly does this and stores the result in combined?

easy

A. SUNIONSTORE combined setX setY setZ

B. SUNIONSTORE combined setX setY

C. SUNION combined setX setY setZ

D. SUNIONSTORE setX setY setZ combined

Solution

1. Step 1: Recall SUNIONSTORE usage for multiple sets

   SUNIONSTORE can take multiple keys after the destination key.

2. Step 2: Verify correct order and keys

   Destination is combined, followed by setX, setY, and setZ.

3. Final Answer:

   SUNIONSTORE combined setX setY setZ -> Option A

4. Quick Check:

   Destination first, then all sets = B [OK]

Hint: List all sets after destination in SUNIONSTORE [OK]

Common Mistakes:

• Using SUNION instead of SUNIONSTORE to store
• Placing destination key last
• Omitting one or more sets

4. Given the following Redis sets:
SADD set1 a b c
SADD set2 b c d
What will be the members of resultSet after executing SUNIONSTORE resultSet set1 set2?

medium

A. ["a", "d"]

B. ["a", "b", "c"]

C. ["a", "b", "c", "d"]

D. ["b", "c", "d"]

Solution

1. Step 1: Identify members of each set

   set1 has {a, b, c}, set2 has {b, c, d}.

2. Step 2: Compute union of both sets

   The union is {a, b, c, d} combining all unique members.

3. Final Answer:

   ["a", "b", "c", "d"] -> Option C

4. Quick Check:

   Union of set1 and set2 = ["a", "b", "c", "d"] [OK]

Hint: Union combines all unique elements from both sets [OK]

Common Mistakes:

• Forgetting to include all unique elements
• Confusing union with intersection
• Assuming order matters in sets

5. You run the command SUNIONSTORE result set1 set2 but get an error. What is the most likely cause?

medium

A. SUNIONSTORE does not accept more than two sets.

B. You forgot to specify the destination key.

C. The sets set1 and set2 do not exist.

D. The key result already exists as a string, not a set.

Solution

1. Step 1: Understand data type requirements

   SUNIONSTORE requires the destination key to be a set or not exist.

2. Step 2: Identify error cause

   If result exists as a string or other type, Redis throws a type error.

3. Final Answer:

   The key result already exists as a string, not a set. -> Option D

4. Quick Check:

   Destination key type mismatch causes error [OK]

Hint: Destination key must be a set or not exist [OK]

Common Mistakes:

• Assuming missing keys cause errors (they don't)
• Thinking SUNIONSTORE limits number of sets
• Ignoring key data types